The present invention relates to a variable fiber optic attenuator and, more particularly, to the use of a silicon optical bench arrangement that can be well-controlled to provide the desired degree of attenuation.
In many optical communication systems, as well as in testing environments, the need arises to be able to adjust the power level of an optical signal. In optical fiber amplifiers, for example, the power level of the optical pump signal needs to be well-controlled to avoid saturating the amplifier. Other systems may require the ability to monitor the power level to avoid power saturation or preserve a required bit error rate (BER). One function of attenuators is to reduce the intensity of optical signals entering a photosensitive device to preclude device damage and/or overloading. In other applications, attenuators may serve as noise discriminators by reducing the intensity of spurious signals received by an optical device to a level below the device response threshold. In still other applications (primarily in testing situations), attenuators are used to simulate the operation of optical systems without the need for long lengths of optical fibers.
There exists in the prior art many diverse arrangements for providing optical attenuation. One such arrangement provides the capability to switch between a pair of discrete power levels. Such an arrangement, while useful in certain circumstances is, of its nature, limited in application. Other approaches to providing variable optical attenuation require a variety of external components such as lenses, filters, prisms, etc., which thus increase the overall size and complexity (as well as cost) of the attenuator.
Thus, a need remains in the art for a fiber optic attenuator that is relatively simple to implement, yet is capable of providing a variable attenuation of the output signal level.
The need remaining in the prior art is addressed by the present invention, which relates to a variable fiber optic attenuator and, more particularly, to the use of a silicon optical bench arrangement that can be well-controlled to provide the desired degree of attenuation.
In accordance with the present invention, a variable fiber optic attenuator is provided utilizing a first, movable fiber and a second, fixed fiber. Both fibers are held in a common fixture that is capable of providing axial alignment between the fibers. The movable fiber includes an arrangement for applying a force to physically move the fiber endface, such as a magnetic (for example, permalloy) sleeve disposed around the fiber with an external electromagnetic magnetically coupled to the sleeve. Without the application of an external force (e.g., magnetic field), the fiber endfaces are in alignment, providing essentially 100% power coupling between the movable fiber and the stationary fiber. Upon the application of an external force, the endface of the movable fiber is displaced with respect to the endface of the stationary fiber so as to reduce the coupling therebetween. In accordance with the present invention, variable attenuation can be achieved by adjusting the strength of the applied force (i.e., magnetic field).
In a preferred embodiment of the present invention, both the movable fiber and the stationary fiber are supported by a silicon substrate including a V-groove for holding the fibers. An additional cavity is formed underneath the movable fiber in the vicinity of the permalloy sleeve to allow for the movement of the fiber. Upon the application of an external magnetic field, the section of movable fiber surrounded by the permalloy sleeve will move downward into the cavity. This downward movement will cause the endface of the movable fiber to move upward and out of alignment with the fixed fiber endface, thus decreasing the optical power coupling between the movable fiber and the endface of the fixed fiber.
In an alternative embodiment, a lid member may be disposed above the fiber-holding substrate. In this embodiment, the application of an external magnetic field causes the endface of the movable fiber to draw backwards, along the optical axis, away from the fixed fiber endface. The resulting separation between the endfaces, therefore, is sufficient to provide the desired optical attenuation.
Other and further embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.